As Tony said, Ethernet-Wireless adapters do exist and are expensive.

A cheaper approach is to get one of the APs that can be setup to act as a normal wireless client, this is different to the AP-AP bridging mode. Not all APs can do this.

Two APs that can do this are the Linksys WAP11 and the Netgear ME102 (which both use the same hardware internally). Neither have this ability with the firmware that ships with them, but an upgrade to the latest firmware enables the client functionality. The Linksys (in the UK at least) is 50% more expensive than the Netgear, but Linksys seem to keep more up-to-date with their firmware. However, as the hardware is the same you can actually get the cheaper Netgear and flash it with the Linksys firmware.

As far as I can see these APs give exactly the same features as a Ethernet-Wireless adapter, but for a half to a third of the price. The only disadvantage I can see is that the APs are a bit bigger than the adapter.

The Netgear does not have an external antenna socket, don't know about the Linksys.